spatial changes in surface sediments derived from the different sediment sources and land uses at...

$: Spatial changes in surface sediments derived from the different sediment sources and land uses at \"El Jable\" (Lanzarote, Spain)$
Journal of Coastal Research SI 48 29-34 III SCCG (Proccedings) Spain ISSN 0749-0208

Spatial changes in surface sediments derived from the different sediment sources and land uses at “El Jable” (Lanzarote, Spain).

L. Cabrera†, I. Alonso† and J. Alcántara-Carrió‡ †Dept. Física ‡Dept. Ciencias Experimentales Univ. Las Palmas de Gran Canaria, Univ. Católica de Valencia 35017 Las Palmas, Spain 46003 Valencia, Spain [email protected], [email protected]@dfis.ulpgc.es

ABSTRACT

CABRERA, L., ALONSO, I. AND ALCÁNTARA-CARRIÓ, J. 2006. Spatial changes in surface sediments derived from the different sediment sources and land uses at “El Jable” (Lanzarote, Spain). Journal of Coastal Research, SI 48 (Proccedings of the 3rd Spanish Conference on Coastal Geomorphology), 29-34. Las Palmas de Gran Canaria – Spain, ISSN 0749-0208. This paper describes the sedimentological properties of sedimentary materials along a North - South orientated sand strip (21 km long and 5 km wide on average) that crosses the island of Lanzarote (Canary Islands, Spain). The area is characterized as a sand sheet because it is mostly covered by aeolian sediments, which were blown southwards from the northern coast. This predominantly aeolian environment is interspersed with agricultural areas, volcanic cones and lava fields. Different sectors of the sand sheet can be distinguished on the basis of grain size properties and carbonate content of the surface sediments. Results are discussed and interpreted considering the geological origin, the human use and the sediment sources of the study area.

ADITIONAL INDEX WORDS: Sedimentology, grain size, carbonate content, sand sheet, Lanzarote

INTRODUCTION Grain size analysis has been widely used to distinguish between

different sedimentary environments, as well as to give information on the processes of transport and deposition (FOLK AND WARD, 1957; FRIEDMAN, 1961; DONGHUAI ET AL, 2002; ABUODHA, 2003). Carbonate content analysis of the sediments gives information on the sediment source of these materials (MAGARITZ and JAHN, 1992). A combination of these textural and compositional properties of the sediments has been used to assess the aeolian sediment availability (ALCÁNTARA-CARRIÓ and ALONSO, 2001).

Lanzarote Island began to form during the Miocene, through volcanic activity in the shield volcanoes of Los Ajaches in the south and Famara at the northernmost part of the island (COELLO et al, 1992). Both shield volcanoes were developed as independent volcanic islands (CARRACEDO and RODRIGUEZ BADIOLA, 1993) and were subsequently connected by eruptive products from a central rift during the Pleistocene (CARRACEDO et al, 2002) (Figure 1). Subsequent aeolian activity gave rise to development of extensive loess and wind-blown sand deposits. The aeolian sediments from this period are locally called ‘jables’, which cover wide lowlands of the central rift, southwards of Famara Beach. For this reason the area is called El Jable.

Detailed studies of sediment transport processes have been carried out in the aeolian deposits located at Fuerteventura (CRIADO, 1987; ALCÁNTARA-CARRIÓ, 2003; ALONSO et al, in press) and Gran Canaria (HERNÁNDEZ, 2002), but not in Lanzarote.

The aim of this paper is to characterize the sedimentological properties of the aeolian surface sediments that form El Jable, and particularly their spatial variations in grain size and carbonate

content, in order to correlate these changes with sedimentary processes and land uses.

Figure 1. Geologic map of Lanzarote. Modified from CARRACEDO et al, 2002.

Journal of Coastal Research, Special Issue 48, 2006 29

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Surface sediments at El Jable, Lanzarote

STUDY AREA Lanzarote is the easternmost island of the Canary Archipelago

located only 140 km away from the African coast. It is 60 km long and 20 km wide and elongated in a NE-SW direction, and covers an area of 862 km2. The central part of the island is a lowland called El Jable, which is mainly covered by marine sediments that were blown southwards by the prevailing northern winds. El Jable is 21 km long and the width range is between 10 km at the northern coast and 4 km at the southern coast (Figure 2). Its total extension is 90 km2 and the relief is low, with a maximum height of 200 m excluding several Quaternary volcanic cones. Six of these are located at the northern part aligned along a N40ºE, 5.5 km-long fissure. In the southern half of the area there is another volcanic vent called Monte Mina, 430 m high.

El Jable is confined in the NE by the Famara escarpments, which probably correspond to the scarp of a giant gravitational collapse (CARRACEDO et al, 2002). The southern end of El Jable is occupied by the airport and tourist resorts close to Arrecife, while in the central part there is a lava field corresponding to the 1736 eruption, and wide areas where the main activities are arable farming, goat grazing and aggregate extraction in places where the sedimentary layer is several metres thick (Figure 3). The southern coast comprises sandy beaches, while the north coast is a rocky platform, except at its eastern end, where the extensive Famara beach is located.

The climate is hot and arid (annual average temperature is 20.7ºC and annual average rainfall is <110 mm) (data period 1972-2000, SPANISH METEOROLOGICAL INSTITUTE), with great seasonal variability. Trade winds are dominant, blowing from the NNE between May and September. During the rest of the year wind is very variable, both in speed and direction.

SEDIMENT SOURCES Variation in the composition of sediments in El Jable indicates

that there is a combination of different sediment sources. Sandy

carbonate-rich sediments in El Jable are clearly marine bioclasts, with a high proportion of calcareous algae, bryozoa, foraminifera, echinoderm spines and remains of other marine organisms.

The SPANISH MINISTRY OF ENVIRONMENT (2002) points out that there are no significant areas covered by sands on the northern continental shelf except in the eastern side, where there are deposits with medium and fine sands. These sediments are transported toward Famara beach and are blown southwards to the inner part of the study area.

These marine materials that cover El Jable are interbedded with clay-rich paleosoils rich in hymenoptera nests and land snails, and fragments of carbonate crusts. This interbedding reflects the Pleistocene paleoclimatic changes that have happened on this geographical region (MECO 2003).

Apart from this main shelf source of sediments, there are additional sediments sources in El Jable, including sediments derived from erosion of the Famara escarpments, which are the northeasterly limit of the study area. These basaltic escarpments are up to 670 m high at Peñas del Chache peak, the highest point of Lanzarote. On its slopes there are small gullies along which sediments are transported during rainy conditions and deposited in the NE zone of El Jable. Moreover, the natural erosion of the superficial paleosoils also contributes to the surface sediments.

Another sedimentary contribution is provided by pyroclastic materials derived from nearby volcanic eruptions and erosion of the volcanic cones and lava fields.

In addition, there are also contributions due to human uses, in particular those related to farming activities. To prepare the ground for agriculture, the farmers usually mix the surface sand with external soils rich in fine sediments and volcanic ashes in order to improve soil humidity (BETANCORT AND GONZÁLEZ, 1990). Furthermore, during the ploughing process, the upper paleosoils are broken and their materials are mixed with the surface sands.

A portion of the total fine sediments founded in the study area is associated with Saharan dust blowing to the Canary Islands by easterly winds. Quartz is rare in the Canary Islands due to its volcanic origins (GÖTZE et al, 1998), however, several authors have reported the aeolian transport of quartz to the Canary Islands from different Saharan source areas (MIZOTA and MATSUHISA, 1995; CRIADO and DORTA, 2003). Penedo

Bay

El JableFam

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Escarp

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2 Km

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El JableFam

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Penedo Bay

El JableFam

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El JableFam

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Arrecife Town

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Figure 2. Location map of the study area

Figure 3. Aggregate extraction in the NW of the study area.



METHODS

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A) B)

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Figure 4. Spatial distribution of the grain size and composition characteristics of the samples. A) Mean size, B) Sorting, C) Skewness and D) Carbonate content (%). Grain size parameters by Folk and Ward (1957) graphic method.

One sampling survey was undertaken in order to collect and describe the surface sediments of El Jable. Due to the dimensions of the study area, the sampling strategy was designed and carried out according to a regular sampling grid, with approximately 1 km between consecutive sampling points and a total of 108 samples (Figure 4). Additional samples were taken from the north and south coasts. All these sampling points were located with GPS. Only the first few centimeters of sediment were colleted in each sample.

Each sample was dry sieved at 0.5 Φ intervals and their grain size parameters were determined using GRADISTAT spreadsheet (BLOTT and PYE, 2001). The carbonate content was obtained for all the samples using the Bernard volumetric method (GUITIÁN and CARBALLAS, 1976). Spatial distribution of every parameter was mapped with SURFER 8.00 software.

RESULTS Surface sediments in the study area are mostly medium and fine

sands (1-3 Φ), while only 5 samples are coarse and very coarse sands (<1 Φ). Most of the finer samples are located in the northeastern area, from Famara beach southwards to the central zone (Figure 4a). Sorting values range between well and poorly sorted (0.3 and 2.0 Φ). The most homogeneous materials occur along the same northeastern area as the finer sediments, as well as in the western part of the study area and on the southern coast. Moderately sorted sediments are located along the northern coast, and the worst sorted materials are located in the southern half of the study area and correspond to samples collected closer to the volcanic vents and lava fields (Figure 4b).

Most of the samples are negatively skewed, except in the western sector, in the northern coast and in a small area close to Famara beach, where symmetrical and positively skewed samples occur (Figure 4c).

Carbonate content shows a clear NW-SE gradient. Samples collected at the northern coast and western sector are very rich in carbonates, with values ranging between 75 and 90%. Carbonate content decreases gradually towards the east and south, with lower values of 15%. The southern coast shows a slight increase, with values between 40 and 55 %. (Figure 4d).

The north coast of El Jable presents a rocky platform with an average of height of 4 meters, where there are very few and small beach deposits. The beach sediments comprise basaltic boulders and coarse sand. On the eastern side there is a large sandy beach called Famara beach. The beach orientation, being exposed to the prevailing wind and waves, and the gently sloping bathymetry of the area, favor the transfer of sediments across the beach towards the inner part of the study area (Ministry of the Environment, 2002).

Carbonate content along the northern coast shows a clear difference from east to west (Figure 5 and Table 1). Samples 1 to 5 in the west, record higher carbonate content in the aeolian environments (jable) than in the intertidal area, while the eastern samples (samples 6 to 11 collected at Famara beach) show the opposite relationship. Samples 1 and 1B have similar carbonate content because the height of the rocky platform is low and the sediments from the upper part fall down into the beach.

On the basis of the grain size and carbonate content of the surface sediments, the study area can be subdivided into six different sectors (Figure 6a).

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Figure 5. Location of samples collected along the north coast.



Table 1. Carbonate content for samples located in Figure 5.

Intertidal beaches Aeolian environments (jables)

# of sample 1B 3B 5B 6B 7B 10B 11B 1 2 3 4 5 6 7 8 9 10 11

% of carbonates 86 40 61 86 86 63 59 79 86 83 88 85 74 58 55 57 41 27

Sectors 2 and 3 present the most homogeneous sediments,

generally of fine sand size. The main mode of each one is 3 and 2 Φ respectively (Figure 6b). Sediments from sector 3 are positively skewed (Figure 6b), while those from sector 2 are mainly negatively skewed except for a zone adjacent to Famara Beach, where they are symmetrical. Sector 3 has higher carbonate content than sector 2.

Sectors 1 and 4 are in general made up of medium sands with

moderate/poor sorting. In both cases, the grain size distribution is polymodal (Figure 6c), which indicates that these sediments either came from different source areas and/or have had different methods of transport or deposition (ASHLEY, 1978; SUN et al., 2002). Sector 1 is more carbonate-rich, more symmetrical and more positively skewed than sector 4.

Samples from sectors 5 and 6 (beaches in north and south coasts respectively) are very different to each other.

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Figure 6. Sectors of El Jable (a) frequency distribution (%weight) of the sectors 2-3 (b), 1-4 (c) and 5-6, beaches on the north and south coast respectively (d).



DISCUSSION AND CONCLUSIONS The differences between sectors in El Jable suggest a difference

in sediment sources and/or sediment transport processes. There are three types of carbonate contribution to the sediments: bioclastic materials of marine origin, bioclastic materials of terrestrial origin and carbonated crusts formed from the dissolution and later precipitation of biogenic carbonates. These carbonated crusts are very thick and abundant in certain areas of the Canary Islands (ALONSO-ZARZA and SILVA, 2002), but they are quite scarce in the study area (MECO, 2003). For that reason the higher carbonate content in samples has to be attributed to biogenic sediments. Since there are fragments of terrestrial gastropods in nearly all El Jable and its distribution is more or less homogeneous, it is obvious that the differences in carbonate content can only be attributed to the contributions of marine origin.

Due to the coastal morphology, the results of the carbonate content, and grain size parameters, it seems evident that the only contemporary input of marine sediments occurs through Famara beach (sector 5) into sector 2.

The sediments that come from sector 5 (Famara beach) show a slight decrease in the carbonate content toward the inner part of El Jable. This is probably due to the increase of terrestrial contributions, mainly from volcanic products that derive from erosion of the Famara escarpments. The surface sediments in sector 2 are similar to the present marine inputs, while in sector 1 the lack of marine inputs in this zone means that the surface sediments have different characteristics than in sector 2.

The good sorting observed in sector 2, is the result of wind transport of the marine sediments from sector 5 southward, while in sector 1 there are no sediment inputs, so that the surface is continuously eroded by the wind, producing wide deflation areas where the residual sediments are poorly sorted. In addition, there are several eroded volcanic cones in sector 1 which contribute coarse volcanic sediments that are mixed with the jable and produce poorly sorted sediments.

Sector 3 is a zone of aeolian deposits several meters thick (Figure 3). For this reason this sector contains symmetrical or positively skewed sediments with better sorting than those in sector 1.

The wide range grain size, polymodal distributions, poor sorting, negative skewness and low percentage of carbonates in sediments of sector 4, can be attributed to alterations associated with agricultural activities (BETANCORT and GONZÁLEZ, 1990). In addition, this is a narrow zone surrounded by lava fields and volcanic vents that enrich the sediment with volcanic products.

Other authors (ALCÁNTARA-CARRIÓ and ALONSO, 2001) have demonstrated that carbonates are preferentially transported by aeolian activity due to their lower density compared to the volcanic products. In this case it is probable that the mix of the aeolian sediments (jables) in sector 4, with terrigenous products as a result of the ploughing processes and the erosion of the near volcanic cones and lava fields, have a greater effect than the aeolian transport of the sediments from the Famara beach.

An increase in carbonate content in sector 6 is evidence of an input of material from the southern marine deposits to the south coast. The differences in grain size parameters between sectors 5 and 6 (Famara Beach and south beaches) (Figure 5d), could be explained considering that samples from the north coast were collected along the foreshore, and therefore from an area under the influence of waves. On the other hand, samples from the south coast were sampled from above the berm, where only extremely high waves could reach and they therefore may have been sorted by the wind.

ACKNOWLEDGEMENTS The authors express their gratitude to all the colleagues who

have participated in the field trips and laboratory determinations. This paper is a contribution to IGCP-495 project. Funding has been obtained from the Canarian Government through research project PI 2002/008.

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